Babuška–Lax–Milgram theorem

In functional analysis, a branch of mathematics, the Shilov boundary is the smallest closed subset of the structure space of a commutative Banach algebra where an analog of the maximum modulus principle holds. It is named after its discoverer, Georgii Evgen'evich Shilov.

Precise definition and existence

Let ${\mathcal {A}}$ be a commutative Banach algebra and let $\Delta {\mathcal {A}}$ be its structure space equipped with the relative weak*-topology of the dual ${\mathcal {A}}^{*}$. A closed (in this topology) subset $F$ of $\Delta {\mathcal {A}}$ is called a boundary of ${\mathcal {A}}$ if $\max _{f\in \Delta {\mathcal {A}}}|x(f)|=\max _{f\in F}|x(f)|$ for all $x\in {\mathcal {A}}$. The set $S=\bigcap \{F:F{\text{ is a boundary of }}{\mathcal {A}}\}$ is called the Shilov boundary. It has been proved by Shilov that $S$ is a boundary of ${\mathcal {A}}$.

Thus one may also say that Shilov boundary is the unique set $S\subset \Delta {\mathcal {A}}$ which satisfies

1. $S$ is a boundary of ${\mathcal {A}}$, and
2. whenever $F$ is a boundary of ${\mathcal {A}}$, then $S\subset F$.

Examples

• Let $\mathbb {D} =\{z\in \mathbb {C} :|z|<1\}$ be the open unit disc in the complex plane and let

${\mathcal {A}}={\mathcal {H}}(\mathbb {D} )\cap {\mathcal {C}}({\bar {\mathbb {D} }})$ be the disc algebra, i.e. the functions holomorphic in $\mathbb {D}$ and continuous in the closure of $\mathbb {D}$ with supremum norm and usual algebraic operations. Then $\Delta {\mathcal {A}}={\bar {\mathbb {D} }}$ and $S=\{|z|=1\}$.

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See also

• James boundary